1. Field of the Invention
The present invention pertains to devices that measure the quantity of radiation to which an entity has been exposed. More particularly the present invention pertains to a reading system for ascertaining the amount of radiation received by a thermoluminescent material. Thermoluminescent materials are materials which emit light upon being heated.
2. Discussion of the Background
By providing an individual or object with a thermoluminescent (TL) material, e.g., a badge containing a TL material, the radiation exposure of the individual or object can be determined. When TL materials are exposed to a source of ionizing radiation, electrons are freed. The release of the emancipated electrons results in positive charges remaining in the TL material. Thereafter, if the exposed TL material is heated, recombination occurs.
During recombination, the positive, trapped holes are freed and photons are released. The level of radiation exposure, i.e., the radiation dose, is proportional to the number of photons released by the TL material during the heating process. By determining the number of photons released from the TL material, the level of radiation exposure can be determined.
In environments where individuals are exposed to radiation, for example, the medical and nuclear power industries, it is common practice for personnel to wear radiation badges for the purpose of determining the level of radiation exposure. By providing the badges with a thermoluminescent material and by collecting and reading the respective badges, it is possible to determine the dose of radiation to which an individual has been exposed.
Such badges are technically known as thermoluminescent dosimeter (TLD) badges. TLD badges are periodically collected and read to determine the amount of radiation exposure.
The Army Primary Standards Laboratory (APSL) processes over 100,000 thermoluminescent dosimeter (TLD) badges per year. In addition, the APSL has maintained National Voluntary Laboratory Accreditation Program (NVLAP) certification using TLD badges and readers since 1986. Over the years the APSL has noted a number of short comings in the performance of TLD readers. Some of the problems include measurement variation between readers, an inadequate badge temperature monitoring system, the requirement to operate in the frequency counting mode at high radiation levels, and variations in energy levels used to heat the badge elements.
Prior art systems typically use heat lamps or high temperature compressed gas to heat TLD badge elements. Both techniques are plagued by heating profile problems as well as the inability to stabilize the output of the heating source. Both heat lamp and compressed gas systems require photon counting at low radiation levels and frequency counting at higher radiation levels resulting in significant non-linearity problems. The resultant saturation of photons at higher energy levels has created significant signal interpretation problems in prior art systems.
Concerns with non-uniform heating and temperature control in TLD reader designs have necessitated the frequent use of quality control measures and the development of complex algorithms in an attempt to rectify inherent design shortcomings.
A prior art system currently utilized is demonstrated in FIG. 1. The prior art system uses the output of a tungsten lamp 10 modified by a silicon filter 12 to heat the TL materials or badge elements 14. Four circular badge elements are demonstrated in FIG. 1. These elements are heated for purposes of determining radiation exposure.
The output spectrum for the tungsten lamp 10 is continuous from about 250 to 3000 nanometers (nm). The peak spectral output for the lamp depends on the operating current and voltage. The peak emission for the tungsten lamp 10 is in the 850 to 1000 nanometer (nm) region. The silicon filter 12 allows radiation at wavelengths of greater thean 1000 nm to be transmitted and used to heat the badge elements. The silicon filter has a peak transmittance of about 91% at 1500 nm and the transmission steadily drops off until it is only about 65% at 3000 nm.
The lamp power of the prior art system is allowed to vary by as much as ±20% from a defined reference value before an error message is generated and the reader is stopped.
A flux sensor (not shown) is mounted in the lamp housing. The output from the flux sensor is used to infer the temperature of the badge elements 14, which as a result of the TL material of which they are composed, function as dosimeter elements. The photons emitted by the badge elements pass through a blue filter (not shown) which is located at the front of the photomultiplier tube (PMT) 30.
The blue filter has a peak transmittance of about 68% at 397 nm and has a full width at half maximum (FWHM) of 122 nm. Li2B4O7:Cu badge elements have a peak photon emission at 370 nm with a FWHM of 60 nm, while CaSO4:Tm badge elements have a peak photon emission at 442 nm with a FWHM of 35 nm.
The prior art system (FIG. 1) uses a PMT having a spectral response of 300 to 650 nm. The high voltage for the PMT is allowed to vary by as much as ±5% from a defined reference value before an error message is generated and the reader is stopped.
With reference to FIG. 1, the prior art TLD reader system includes a magazine 18 which holds the radiation badges or dosimeter holders 20 that contain badge elements 14. An automated slide 22 is used to extract element plates 24 which contain TL materials or elements from each respective badge 20 stored in magazine 18. Slide 22 is provided with a reference aperture 26. An ID code reading unit 28 identifies the individual associated with each respective badge.
Upon the badge elements 14 being heated, photons are emitted from the TL material and are filtered by the blue filter (not shown) before proceeding to the photomultiplier tube PMT 30. A CPU (not shown) connected to PMT 30 processes the collected information.
FIG. 2A, FIG. 2B, FIG. 2C and FIG. 2D demonstrate the number of hits, i.e., number of photons detected, as a function of time for the prior art system of FIG. 1. FIG. 1 depicts four TL badge elements 14 which are associated with each radiation badge or dosimeter holder 20, the badge elements being positioned on the element plate 24. Two of these badge elements are Li2B4O7:Cu badge elements and two of the badge elements are CaSO4:Tm badge elements. FIGS. 2a and FIGS. 2b graphically represent hits as a function of time for the two respective Li2B4O7:Cu badge elements, and FIG. 2c and FIG. 2d represent hits as a function of time for the two respective CaSO4:Tm badge elements.
A number of problems have been recognized with the prior art TLD reader system of FIG. 1. The tungsten lamp used as the heating mechanism, in conjunction with the silicon filter, is inefficient resulting in a considerable amount of wasted heat. In addition the allowed tolerances in lamp power lead to variations in the heating profiles of the badge elements, and the use of the flux sensor has proven unreliable in determining the temperature of badge elements.
Another area of concern is the transmission characteristics of the blue filter. The blue filter's transmission range is too narrow to cover the emission peaks of badge elements, especially in the case of Li2B4O7:Cu badge elements.
Still further, in that the emission of badge elements is hemispherical in nature, the emission geometry of the emitted photons has not been properly taken into account in the design of prior art photon collectors. The result has been significant signal loss and inefficiency.
The non-linearity and saturation characteristics of the PMT force the prior art TLD readers to be operated in both the photon and frequency counting modes. Operation in the frequency counting mode results in a reduction in measurement accuracy.
For example, the PMT used in the prior art system when operating at high voltage levels allows high tolerances in the signals processed. These high tolerances, associated with the frequency counting mode, equate to gain variations which approach a factor of nearly 2.6.
Due to the close proximity of the badge elements to both the silicon filter and the blue filter, the filters must be cleaned on a regular basis to avoid serious system performance problems.
Further, the prior art optical design is not hardened for purposes of field operations and would be unlikely to be able to support deployable missions.